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360                               CHEMICAL ENGINEERING
Heat Transmission.In order to transmit heat from one body to the other there must necessarily exist a temperature difference, and generally speaking the capacity of an evaporator depends on the temperature difference between the heating steam and the boiling liquid.
A number of conditions influence the heat transmission.
1.  Material, thickness and cleanness of heating surface.
2.  Temperature difference depending on temperature of heating steam and boiling liquid, also density, viscosity, total depth and circulation of the liquid.
3.  Velocity, distribution, density and quality of the steam.
The heat to be transmitted through a metal surface has to overcome three resistances:
1.  The entrance from the steam into the metal surface.
2.  The passage through the metal.
3.  The exit from the metal into the boiling liquid.
These factors have been determined by numerous experiments, and the average results are given in L. S. Marks7 "Mechanical Engineers Handbook," pages 303-306, as follows:
fci = 2,000; denotes coefficient of conductance of steam film. Jc2 = 1,000; denotes coefficient of conductance of boiling liquid film. K = coefficient of thermal conductivity of the plate for a thickness of 12 in. (see L. S. Marks: Tables No. 19, 20, 21).
K/b  coefficient of conductance of plate for a thickness of b in inches. R = resistance to the flow of heat through films and plate, fco = 1/.R = total sum of conductances. h and tz  temperature of steam and boiling liquids.
All factors are given per degree Fahrenheit, and Q is the total heat in B.t.u. transmitted per hour per square foot.
For an evaporator where steam and boiling liquid are separated by a metal surface, Q = fcofti - fe), and R = 1/fci + (l/fca) + (b/K).
For a copper tube, K = 200. The thickness of the tube is usually He in.; therefore b = H92, and with a steam pressure of 5 Ib. and a vacuum of 26 in., the total temperature difference would be 102F. Under these conditions, R = (Mjooo) + M;OOO) + (1/(192 X 200), Q = 1/655. Q = 655 X (*i - fc) = 66,810 B.t.u.
This equation shows that with other conditions equal, the thickness of the tube has very little influence on the transmission of heat, and with a copper tube J in. thick, the amount of heat transmitted would be 66,300 B.t.u. For an iron tube KG in. thick, and K = 35, Q would be 61,812 B.t.u. The small theoretical advantage of copper over iron is also true in actual practice, except that usually iron tubes are more easily corroded than copper tubes, and the corrosion forms an insulating film on the iron which greatly reduces the heat transmission. For practical purposes iron of the same thickness will transmit'about 80 per cent of the heat transmitted by copper.
For lead tubes K in. thick, and K = 20, Q would be 40,086 B.t.u.
Scale will exercise considerable influence on the amount of heat transmitted. Correct data for K are not known in this case, but may be taken the same as boiler scale, which will average about 1.50. The total resistance will be the sum of the resistance of the clean tube and the resistance caused by the scale. Therefore, for a J-f 6-in. copper tube coated with a K2 in. lime scale, R = ^55 4- (l/(384 X 1.50), = KooJ Q is 30,600 B.t.u., that is about 50 per cent of the heat transmitted through a clean copper tube. This reduction in heat transmission is somewhat higher than in actual practice, which is probably due to the fact that in most cases scale is not very dense and will permit direct contact between the metallic surface and the boiling liquid. Incrustations of salt and organic matter have a similar influence, and it happens